Problem when warmed up....

[buickwagon]

Where is the PCM grounded or which wire is it?

I'm out of town today, and don't have my manual with me to check. I'll try to track that down for you tomorow.

The VSS is fine, as a brand new one was put in and it didnt change any of the symptons. The fact that sometimes it works fine and sometimes not, is really frustrating. I have replaced the wiring directly from the PCM to the VSS and theres no change... I will try connceting a test light this evening. Will I need the car running or the key in the on position at least?

Key on, engine off. I'm not surprised that replacing the sensor didn't work. The problem with engine codes is that they represent what the computer thinks is the problem. All too often there are other things that can mimic similar symptoms. When you have multiple codes and other symptoms like this, it's usually a matter of analyzing the systems and looking for the common denominator.

In your case, having replaced the wires to the sensor pretty much confirms the problem is elsewhere -- either in the PCM or the wires to it. And IIRC, you replaced the PCM with a known good unit already, correct? The test I suggested will hopefully further isolate the problem to either the ground circuit or the power circuit.

Out of curiosity, why did you replace the PCM? Was this part of your diagnostics, or something related to your modifications (torque converter, etc.)?

 

[frostydevil]

I picked up a used PCM out of a '95 that had a frozen motor from sittin too long, for like $40 at a local yard here. For that price, I just swapped the PCM to eliminate it from the equation. It still acts the same. I am on the same road you are and thank you much for your help with this. A power or grounding issue, and I'm tearing my hair out cause I cant find it. If I cant figure it out, I'm probably going to end up selling it to Cory LOL. I will be doing that test and maybe more diagnostics this weekend hopefully, hook the scanner up again and see if any readings are funky this time. If it was a grounding issue at the PCM, wouldnt it affect other systems as well and not just the control of the tranny? I have no other issues with any system besides this at the moment. (notice I had to say "at the moment" LOL)

 

[buickwagon]

Ok, first off, I thought you are having a bunch of issues -- didn't this thread start off with an engine miss under load?

Second, you have a 94, right? You put a 95 PCM in it? Did you reprogram it for 94 specs? I believe those two transmissions are different -- the 95 introduced the pulse-width control for the torque converter lock-up, so it has an extra solenoid. Might explain why your codes changed, if nothing else.

Third, there's several "ground" connections associated with the PCM. Some ground the PCM itself, some provide grounds for other circuits it monitors. For example, connector B, cavity 16 seems to ground the transmission temp, engine coolant and MAP sensors through the PCM. Strange things can start happening when grounds disappear -- systems try to ground through any circuit with a lower voltage potential -- so a poor connection between the PCM and where those 3 join could cause all 3 to act up without necessarily affecting any other system or even setting any codes.

PCM grounds include:
Connector A (red), Cavities 2 (black/white), 18 (black/white)(both ground the PCM itself), 31 (grounds the VSS through the PCM)

Connector B (black), Cavities 6 (grounds AC pressure, IAT, TP), 16 (purple) (MAP, ECT, Trans. temp)

Connector C (grey), Cavity 32 (black/white) (grounds PCM itself)

Connector D (blue), cavity 1 (black/white) (PCM)

All the above (black/white) PCM grounds connect back to G104 -- the front of the left hand cylinder head.

 

[frostydevil]

Ok, first off, I thought you are having a bunch of issues -- didn't this thread start off with an engine miss under load?

Yes it did. But I have come to believe this is related to my gremlin.

Second, you have a 94, right? You put a 95 PCM in it? Did you reprogram it for 94 specs? I believe those two transmissions are different -- the 95 introduced the pulse-width control for the torque converter lock-up, so it has an extra solenoid. Might explain why your codes changed, if nothing else.

Of course I didnt spend close to $200 just to reprogram it. A '95 obd1 pcm will work with a '94 without having to reprogram it. Yes thats maybe why the code changed, but it doesnt care that the solenoid isnt there. It sends the signal regardless, but it doesnt go anywhere.

Third, there's several "ground" connections associated with the PCM. Some ground the PCM itself, some provide grounds for other circuits it monitors. For example, connector B, cavity 16 seems to ground the transmission temp, engine coolant and MAP sensors through the PCM. Strange things can start happening when grounds disappear -- systems try to ground through any circuit with a lower voltage potential -- so a poor connection between the PCM and where those 3 join could cause all 3 to act up without necessarily affecting any other system or even setting any codes.

PCM grounds include:
Connector A (red), Cavities 2 (black/white), 18 (black/white)(both ground the PCM itself), 31 (grounds the VSS through the PCM)

Connector B (black), Cavities 6 (grounds AC pressure, IAT, TP), 16 (purple) (MAP, ECT, Trans. temp)

Connector C (grey), Cavity 32 (black/white) (grounds PCM itself)

Connector D (blue), cavity 1 (black/white) (PCM)

All the above (black/white) PCM grounds connect back to G104 -- the front of the left hand cylinder head.

This is extremely helpful!! Thank you!

 

[buickwagon]

I came across these posts on the Impala SS forum while searching to see if there were any possible issues with using that PCM and came across these. Do they sound at all familiar?

10. Trans shifts into gear harshly, car feels sluggish off the line, No 1st, 4th or TCC lockup available, Manual 2nd, 3rd and Reverse are only available gears, CEL is on: Transmision is either in limp-home mode or has lost electrical power. If there are lots of error codes in the PCM, check the underhood fuse that powers the transmission, and if it pops again, look for a short in that circuit like an O2 sensor harness touching exhaust. Otherwise, check PCM codes for a particular fault in the transmission causing the PCM to put it in limp-home mode.

Just had Shane install a 3000 converter and now my car goes into limp mode. Well, not all the time, but if I hammer it and spin the tires through 1st and into 2nd, then it happens. Fluid is good, EMI fuse is good, no check engine light, so I'm assuming it's a VSS dropoff problem. Shut the car off and let it sit for a while and it's fine, as long as I don't drive it too hard.

(BTW sounds like you can use the 94 tranny in the 95 car without reprogramming. It will work but throw a silent code 83 but unless there's an engine issue, I don't see any reason you can't use the 95 PCM in the 94 car).

 

[frostydevil]

I dont have a blown fuse for the tranny, and 02 sensors are working fine. Wires are ok too. There are no other engine codes than what I have already stated.
I do have a slightly higher stall with the corvette TC, and I did look into the VSS dropoff problem, but my symptons are different. It doesnt just do it when I hammer on it. If its acting up, its acting up and doesnt matter whether or not I'm hard on it.
I'm going to check and recheck and clean and re-clean every ground I can find and know about. What frustrates the hell outta me, sometimes, once in awhile everything works fine, but most of the time, it acts like there is no power to the tranny, as a scanner will not be able to pick up the signal from the VSS, which is what controls alot of how the PCM commands the tranny, not to mention the speedometer in the dash. I will also try and see if anything else is affected, such as MAP sensor, TPS, MAF, or anything else thats not acting normal. - (that could explain the feeling of an engine miss or running rough while applying power and could narrow down which ground or power problem I have)

 

[frostydevil]

I never thought I'd be so distrught to say this :
.....the car has been working fine the past few days.

Grrrrrr!

 

[frostydevil]

Well I think the problem has been solved and the gremlin has taken a vacatiobn (for now). I really think it was the last time I re-did the pcm grounds to the block. Tiz the only thing I can think of. Anyway, has been working good!!

 

[BiG_TaNk]

Nice , Glad it worked out for you.. So is it safe to say you'll be taking it off craigs list? I always wanted a LT1... lol

 

[frostydevil]

I havent gotten any offers on it whatsoever. Just an interest from Cory if his truck sold, and now you. To me, everything is for sale....

 

[MrBuick (Magnum)]

I've only had two calls on the truck in 3 weeks now... It's quite frustrating. I have the itch for a wagon now. If I end up with this one wagon on the cheap, I'll still be interested if the truck sells. However, if I can't get this wagon on the cheap, all the funds from the truck sale will be put towards buying a wagon. I just can't pass up a DCM LT1 RMS!

 

[frostydevil]

Of course I didnt spend close to $200 just to reprogram it. A '95 obd1 pcm will work with a '94 without having to reprogram it. Yes thats maybe why the code changed, but it doesnt care that the solenoid isnt there. It sends the signal regardless, but it doesnt go anywhere.

Well having said that, I have to say there is a little truth to that. The '95 PCM works, however I DO NOT have TCC lockup with it.

 

[buickwagon]

The '95 PCM works, however I DO NOT have TCC lockup with it.

The important thing is to remember the car is running fine now. Don't lose sight of the positives!!! 😛

You mentioned earlier that it would cost you around $200 to have the PCM reprogrammed. I found a program from an outfit called "tunercat" that claims to give you the capability to reprogram 94/95 LT1 cars through the OBD I connector. It is around $160.

Or you could just swap in your original PCM and see if it works now that the grounds are cleaned. 😉

 

[frostydevil]

I've looked into tunercat, and more often than not, you can find the program for free. However, the cable to hook up your laptop to the OBD1 port is around $300 last time i looked (few years ago) That definitely would be the way to go LOL! That also would mean Id have a new toy to play with - hehe. Reason I said close to $200, was because, IIRC,pcmforless.com charges $175.

Yes it is important to look at the fact that it is running really good right now. Low14's 1/4 mi (according to my smartphone app) Theproblem ended up happening again yesterday, so I brought it into the garage and replaced the wires from the PCM to ground. One of the wires looked like it was frayed and corroded right at the pin for the terminal to the PCM. I think that might have been my problem. WHAT A BITCH to find FFS!!

I did swap in my '94 PCM after I noticed no TCC lockup. I just had to post that comment to my previous statement regarding the '95 pcm in a '94. Yes, everything works fine with that swap, except TCC lockup.

Thank you Buickwagon! You have been a big help in finding my gremlin!

 

[heath_94RM]

Has anyone used Solomon? He has a cable loan method and the cheapest prices I have seen.

http://www.lt1pcmtuning.com/

 

[turbostar]

Solomon is outstanding.He reprogrammed my 95 pcm when I put that drivetrain in my 65 pickup.

 

[MrBuick (Magnum)]

I've looked into tunercat, and more often than not, you can find the program for free. However, the cable to hook up your laptop to the OBD1 port is around $300 last time i looked (few years ago) That definitely would be the way to go LOL! That also would mean Id have a new toy to play with - hehe. Reason I said close to $200, was because, IIRC,pcmforless.com charges $175.

What? The cables sell for about $75.

http://www.akmcables.com/ic.htm

 

[frostydevil]

Holy F*&K!!!!! Did I miss something? I could have swore a few years ago they were going for an astronomical amount of money. I thought thats what the catch was with the software being shareware basically. Never looked into it again. Just doing a quick search, I've found USB ALDL OBD1 cables for even cheaper. $60 here - Damn! Now I have a use for my old laptop. I know what my next purchase is gonna be!! LOL Thanks Cory! (son-of-a-biotch! where am I going to find time to play around with this LOL!)

http://www.aldlcable.com/sc/details.asp?item=aldlobd1u

 

[TheMasterPing]

I have the cable Frosty linked and have been happy, also i've used Solomon (LT1PCMtuning.com) and had a good experience.

 

[BiG_TaNk]

I've wanted that aldl cable for a couple months now but I don't have a laptop so it would be useless....